In the normal human heart, the sinus node, that is generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. Disruption of the heart's natural pacemaker and conduction system, as a result of aging and/or disease, can be successfully treated using various implantable cardiac stimulation devices, including pacemakers and implantable cardioverter defibrillators. A pacemaker is generally arranged to deliver rhythmic electrical pulses to the heart to maintain a normal rhythm in patients having bradycardia, which is too slow of heart rate, or other conduction abnormalities. In contrast, an implantable cardioverter defibrillator, commonly referred to as an “ICD”, can recognize tachycardia and/or fibrillation and deliver electrical therapy in order to terminate such arrhythmias. In addition, such ICDs may often be configured to perform pacemaking functions (or pacing) as well.
Depending upon the patients' needs, the ICDs generate pacing, cardioverting, and/or defibrillating pulses, and deliver them to excitable cardiac tissues of the patients' heart by means of implanted electrical leads and electrodes. Since the lead is an essential part of the therapy a lead failure renders the device ineffective.
To detect lead failure, the cardiac stimulation devices, such as ICDs, monitor the impedance of the implanted leads. For high-voltage leads, a sudden rise of the lead impedance above 100 ohms is generally considered a signs of lead failure. To this end, certain conventional stimulation devices incorporate a high-voltage lead integrity check (HVLIC) system that generates a detection waveform, delivers the waveform through the implanted leads, and checks the integrity of the leads based on the delivered waveform parameters measured by the device.
In a conventional method of performing high-voltage lead integrity check a 12V, 3 msec truncated exponential pulse illustrated in FIG. 3, is delivered from the high voltage capacitors through the defibrillation leads. The low amplitude of this pulse is an attempt to get below perception thresholds of sensory and motor nerves and muscle tissue. The 3 msec duration of this pulse is chosen to allow enough time to enable the measurement of a voltage change across the high voltage capacitors of the implantable device. Several problems are associated with the monophasic detection waveform of FIG. 3. First, the duration of 3 msec is generally much longer than the time constants of the sensory and/or motor nerves that are generally on the order of 200 μsec to 500 μsec. In fact, such a duration almost approaches the time constant for stimulation of de-innervated muscles, and could thus cause muscle twitches and sensation.
In addition, the ICD is designed to charge its high voltage capacitors to about 800 volts with 2% accuracy. However, this accuracy degrades when charging to, or measuring lower voltages. This loss of accuracy is compounded by measurement errors associated with even lower voltages, and results in an inaccurate high-voltage lead integrity check of the lead impedance.
Another problem is that approximately 3 volts of the monophasic detection waveform is consumed in polarizing of the electrode-electrolyte interface at the electrodes and body fluids boundary. This voltage drop occurs, for example, between the blood and a polished platinum electrode, obscuring the delivered signal and compounding the inaccuracy of the measurement of the lead impedances.
Accordingly, it would be desirable to provide a more reliable impedance measurement system for the cardiac stimulation device, which need has heretofore remained unsatisfied.